1. Field of the Invention
The present invention relates to a laser measuring apparatus and, more particularly, to the improvement of a mechanism for securing the linearity of laser light against the heat generated in a laser light emitting portion.
2. Description of the Prior Art
Laser light is characterized by coherent monochromatic light with rays in phase with each other and is widely used in the field of a measuring apparatus such as apparatus for measuring the shape of an object from the intercepted region of laser light and the length of an object by the interference of light waves.
These laser measuring apparatuses are premised on the linearity of laser light.
Laser light, however, proceeds in the casing of a laser light emitting portion out of a predetermined optical path, sometimes resulting in generation of a large error in the measured value.
This phenomenon will be explained concretely. The air in the casing of the laser light emitting portion constitutes a medium for laser light. In the casing, heat is generated from a laser light emitting element, a driver circuit, etc. and locally heats, namely, produces what is called a heat island in the medium for laser light. The density of the air in the heat island is different from the density of the air at another portion which is not influenced by heat, thereby producing fluctuation such as heat waves.
That the density of air is different depending upon the locality means that the air as the medium has different refractive indexes depending upon the locality. Therefore, when laser light passes through the heat island, the laser light is refracted in correspondence with the refractive index of the heat island and what is called "fluctuation" is produced. In other words, it is impossible to maintain a predetermined linearity.
When measurement is carried out by a laser measuring apparatus while using an air conditioner in the environment of a high temperature, the chill from the air conditioner is transmitted to the casing of the laser light emitting portion. The cold transmitted to the interior of the casing produces a portion having a different refractive index on the medium for the laser light in the same way as in the generation of a heat island. It is therefore impossible to maintain predetermined linearity sometimes resulting in the generation of a large error in the measured value.
As a countermeasure, a laser measuring apparatus is conventionally known in which the laser light emitting element and/or the casing is covered with a heat insulating material so as to ensure the progress of laser light along a predetermined optical path in the casing of the laser light emitting portion (Japanese Utility Model Laid-Open No. 174210/1987).
In this method of using an insulating material, however, it is difficult to produce a completely insulated state, so that it cannot be said that a satisfactory effect is obtained. Especially, in the case of making the apparatus itself compact by using a semiconductor laser as a laser light emitting element, since the volume of the casing of the laser light is small, the volume occupied by the insulating material is limited, thereby making it impossible to provide many insulating members. In addition, since the thermal diffusion is limited, a more serious problem is produced.
If the means for reflecting the laser light emitted from the laser light emitting element is composed of a rotary mirror, a driving motor for rotating the rotary mirror, and a driver circuit for driving the motor, heat is also generated from the driving motor and the driver circuit, so that it is necessary to consider the influence of such heat generation.
Especially, in a laser measuring apparatus which is required to have a high resolution, the influence of the generation of heat is a very serious problem.